Underpinning

Overview: To picture a structure that has been underpinned, visualize your home’s foundation resting upon underground steel piers. These steel piers safely and permanently transfer the weight of a structure onto solid bedrock or load-bearing strata. Piers are used to stabilize a structure and have the ability to return it to its original elevation.

Function: Underpinning is a repair method used to fix sinkhole damaged structures. It utilizes underground steel piers to stabilize the foundation. Concrete foundations usually shift because the upper level of soil is loose, or because a sinkhole has occurred within the vicinity. However, there is always a level of stratum deeper within the earth that is capable of bearing the load of the foundation.

Steps to Installation: The process of underpinning a foundation involves extending the foundation’s load onto steel piers that extend down to solid bedrock. Piers can often lift structures to their original level. Resistance piers, as they are sometimes called, are effective for extremely large structures as well as smaller residential dwellings.

First, a bracket is installed at each pier location in accordance with the engineer’s plan.

Then, a large steel frame with a hydraulic ram on top slides onto the bracket and pushes 4ft sections of pipe down through the opening in each bracket. The pipes are driven to the bedrock or load-bearing strata, well below the depth of soils affected by seasonal changes in the water table.

After the piers have been installed to the appropriate load-bearing capacity, they are hydraulically jacked to sufficiently support the structure. The brackets are bolted down with a top steel plate leaving a portion of the threaded rods intact.

Upon completion of this phase, all equipment is removed and all of the pier installation areas are filled.

Terminology: A pier is a bracket with a 4ft guide sleeve and 4ft sections of interlocking tubular steel pipes.